(186bd) A Three Dimensional Fluid-Structure Interaction Finite Element Model of Wave Propagation in SAW Device: Application to Bio-Fouling Removal

I. Motivation/Background Non-specific protein binding to the surface acoustic wave (SAW) device surface, during bio-sensing applications, leads to bio-fouling which causes a significant reduction in their sensitivity and selectivity. Acoustic streaming, defined as fluid motion induced from high intensity sound waves, can be used to remove these non-specifically bound proteins and allow reuse of SAW devices. In this study, we report for the first time, a fully coupled three dimensional finite element fluid-solid interaction (FSI) model of the SAW device subject to liquid loading to investigate the streaming velocity fields and forces induced by SAW device. II. Computational details In the current work, a SAW device based on YZ-LiNbO3 with a liquid loading was modeled to gain insights into the acoustic streaming phenomenon (Fig. 1). The dimensions of the piezoelectric substrate were 400 micron width x 500 micron propagation length x 200 micron depth was simulated to gain insights into the acoustic streaming in SAW devices. Two IDT finger pairs in each port were defined at the surface of Y-cut, Z-propagating LiNbO3 substrate. The fingers were defined with periodicity of 40 μm and aperture width of 200 μm. The IDT fingers were modeled as mass-less conductors and represented by a set of nodes coupled by voltage degrees of freedom (DOF). The model was meshed with tetragonal solid elements with four degrees of freedom, three of them being the three translations and the fourth being the voltage. Fluid is modeled as incompressible, viscous, and Newtonian using the Navier-Stokes equation. In modeling fluid-solid interaction, a purely Lagrangian frame is incapable of dealing with strong distortions of the fluid mesh. A purely Eulerian frame for the fluid domain introduces complexity in fluid-solid coupling. Therefore, mixed Lagrangian-Eulerian or Arbitrary Lagrangian Eulerian (ALE) methods are used for kinematical description of the fluid domain. The Eulerian description is used for ?almost contained' flows and Lagrangian description is used for regions where the mesh would be highly distorted if required to follow fluid motion. The mesh is constantly updated without modifying the mesh topology. To achieve bidirectional fluid structure coupling, stress and displacement continuity are maintained across the fluid-structure interface. To achieve this, displacements are transferred from solid to fluid and pressure from fluid to solid. The fluid mesh is continuously updated as the piezoelectric substrate undergoes deformation. The Standard k-ε Model is used to study flow in the turbulent regime. The simulation was carried out for 100 nanoseconds (ns), with a time step of 1 ns. The excitation of the piezoelectric solid was provided by applying an AC voltage (with a peak value of 2.5 V and frequency of100 MHz) on the transmitter IDT fingers. III. Results/Discussion The surface acoustic wave displacement on the SAW device is depicted in Figure 2a. Our simulation results indicate the acoustic wave couples strongly to the liquid medium and radiates longitudinal waves into the liquid medium thereby inducing fluid motion. The transient solutions generated from the model are used to predict trends in acoustic streaming velocity along various locations in a SAW delay path. Parameters studied in this model include intensity, frequency, fluid density, and viscosity. The fluid velocity profiles, shown in Figure 2b, are used to compute the lift and drag forces on the particles which in addition to the direct SAW force and acoustic radiation force act against the particle-surface adhesive forces and act to selectively remove the non-specifically bound proteins. A comparison of these removal forces with the adhesion forces for the specifically and non-specifically bound proteins provides insights into mechanisms for the removal of non-specifically bound proteins from the biosensor surface. The results will be discussed in detail.